Chemistry Reference
In-Depth Information
cooperation between the two heads is necessary for processive movement [46]. When
excess load (greater than stall force) was applied, motors such as kinesin showed
stepwise movement in the backward direction [47].
1.6
Mechano-Chemical Coupling of Molecular Motors
In order to understand the mechanism of molecular motors, it is critical to describe
how mechanical steps are related to ATP hydrolysis (see Chapter 2). Simultaneous
imaging of ATP turnover and mechanical measurements using a single molecule
optical trap was used to determine the mechano-chemical coupling in myosin [48].
In contrast to ATP-driven but irreversible actomyosin motors, the ATP synthase
F
0
F
1
is a reversible mechano-chemical machine. Using single molecule detection,
ATP synthase F
0
F
1
was found to be a rotarymotor (see Chapter 10). The rotation of F
1
was
first observed by monitoring the rotation of
fluorescent actin
filaments attached
to the rotor [49]. The 120
o
rotation unit steps generated by a single ATP molecule
were detected at low ATP concentrations [50]. It was also demonstrated that ATP
is synthesized when F
1
is forced to rotate in the backward direction [51]. This
mechanism has been extensively studied.
1.7
DNA-Based Motors
DNA-based motors translocate along a DNA molecule powered by nucleotide
hydrolysis while they transcribe gene information. As compared with actin- and
microtubules-based motors, the properties of the DNA-based motors are not yet
known. Individual DNA molecules were
uorescent
dyes and using video-enhanced optical microscopy [52]. This led to single DNA
molecules being manipulated by magnetic traps [53], laser traps [54], and by
pipette [55]. These manipulation techniques have been used to measure various
mechanical properties of DNA such as the force
-
extension relationship and its
interaction with motor molecules.
Transcription by a single RNA polymerase molecule has also been observed by
laser trap. The displacement and force exerted on a trapped bead attached to the end
of a DNA molecule being pulled by an RNA polymerase was measured while the
polymerase was immobilized on a slide glass during transcription [56, 57]. In 2005,
RNA polymerase was shown to have a step-size of 3.7 A
, equivalent to a DNA base
pair [58]. This technique also proved that RNA polymerase moved along a DNA
helix [59] as con
rmed by monitoring the rotation of a magnetic bead attached to the
end of a DNA molecule during transcription while the RNA polymerase was
immobilized on the slide glass. Unlike actin
filament- and microtubule-based
molecular motors, some DNA-based motors change the topology of DNA while
they translocate. This change has been used to monitor the movement of DNA-based
first visualized by staining with
